While polymeric films are used in countless applications, the demand is increasing for films incorporating decorative diffraction patterns and holographic images. Such decorative films are commonly used for laminated paperboard and heat sealable flexible packaging. While a variety of films are available for use in such applications, oriented films are frequently preferred due to their improved strength, tear, and barrier properties as well as their enhanced durability and dimensional stability. In particular, such oriented films may be utilized as an embossed, metallized product or as a transfer film which is repeatedly reused to produce a multitude of other embossed, metallized products.
Oriented film is produced by stretching the film along one direction, either the transverse or longitudinal direction, producing uniaxially oriented film, or stretching the film along both directions, producing biaxially oriented films. Biaxially oriented films may be produced by simultaneously stretching the film in both directions or sequentially stretching the film along the transverse and longitudinal directions. Simultaneous biaxially oriented films may be produced using a tentering process or a bubble process whereby a bubble is blown of the film, stretching it biaxially. Sequentially biaxially oriented films are produced by stretching the film in sequential steps, including stretching the film in both directions in each step of a series of sequential steps, stretching the film completely in one direction then completely in the other direction, or stretching the film in a repetitive process by stretching the film first in one direction, then in the second direction, and stretching the film again in the first direction. Sequentially biaxially oriented films are typically produced using a tentering process, whereby a tentering frame comprising moving endless chains grasp the edges of the film, stretching the film along a direction transverse to the direction of film travel. The extent of stretching along the transverse and longitudinal directions in a biaxially oriented film need not be equal.
Uniaxial orientation enhances the physical properties of films in one direction; certain properties in the perpendicular direction are usually unimproved. The orientation temperature, governed by the material determines the final properties. Semi-crystalline polymers are oriented between the glass transition temperature T.sub.G and the melting T.sub.M, whereas amorphous polymers are oriented above T.sub.G. In general, the polymer is oriented by rapid stretching in the temperature ranges as described above, followed by rapid annealing. The latter ensures that the orientation is not lost by molecular relaxation. The degree of orientation is determined by the extension rate and the degree of extension.
Prior art methods of producing surface relief diffraction images on embossed biaxial oriented films utilize a thermoplastic film that has been oriented using either simultaneous biaxial orientation, as with the bubble process, or by sequential biaxial orientation commonly accomplished with drawing and tentering. After the extrusion and orientation of the film, the oriented film is cooled and wound into a roll for transport to a separate area or facility where embossing of the film takes place.
During the embossing step, the film is moved over a heated die roller that presents the moving web of film with the surface relief structure to be replicated. By opposing the die roller with a platen roller, and passing the film between the die roller and platen, sufficient heat and nip pressure can be applied to transfer a diffraction pattern into the film web.
These prior art methods suffered from problems such as distortion of the film web, degradation of the oriented crystalline structure of the film, and inconsistent embossing brightness due in part to the inability to apply uniform heat and nip pressure to the film across film widths wider than 12 inches. While the use of narrow embossing rollers (typically less than 12 inches in length) and narrow web films overcomes some of these problems by permitting the controlled application of uniform heat and pressure, the short embossing rollers resulted in slow speeds, narrow web widths, and low manufacturing yields.
To properly emboss a wider web of film and avoid the prior art problems associated with the application of uniform heat and pressure, the film side to be embossed is first coated with an emboss coating. The emboss coating has a melting point which is lower than the melting point of the film being embossed. Thus, less heat is required to soften the emboss coating for proper embossing. Alternately, a liquid resin may be precoated onto the film web and cured with ultraviolet or electron beam radiation while simultaneously embossing the film.
To achieve a proper diffraction pattern in the subsequent embossing step, the coated film is preheated prior to contacting the embossing die roller, thereby softening the coating while maintaining the properties of the base film. In this manner, dependence on uniform heat and pressure is reduced and wide rollers achieve the necessary level of uniformity of pressure across the wide film web.
If the coated, embossed film is utilized in the manufacture of a metal coated film product, a metallic layer such as aluminum or the like is applied to the emboss coating by vapor deposition or similar application method after the embossing step.
In the prior art, if the film is to be utilized as a transfer film which can repeatedly be reused to produce a multiplicity of metallic, embossed films, a release coating is applied to the transfer film prior to the application of the emboss coating. The release coating forms a weak bond with the surface of the transfer film and forms a strong bond with the emboss coating. A metallic layer such as aluminum or the like is applied over the emboss coating after embossing. A second substrate is then laminated to the previously embossed metallic layer using catalytic or actinic radiation curable adhesive or the like. When this structure of transfer film, release coating, emboss coating, metallic layer, adhesive and second substrate is delaminated, a failure occurs in the weaker bond between the transfer film and the release coating, the emboss coating and metallic layer remaining with the second substrate and adding a high quality, diffractive metallic surface to such substrate. When this structure is delaminated, the embossed coating is split from the transfer film, leaving an unembossed transfer film. Thus, each use of the transfer film requires the additional step of embossing the diffraction imagery into a newly applied coating. Depending on the composition of the film, the oriented transfer film can be reused by reapplying the release coating and emboss coating, embossing the emboss coating, redepositing a new layer of metal onto the resins, and applying an adhesive layer and second substrate. The additional costs incurred by the reapplication of such coatings renders the use of oriented transfer films for many products prohibitively expensive.
An improvement by Watkins et al., described in U.S. Pat. No. 4,968,370 allows the formation of diffraction information in a carrier film by using extrusion casting of thick profile films. Substantially molten thermoplastic resin such as polypropylene is continuously extruded onto a rotating cooling cylinder presenting a surface relief diffraction pattern or holographic image to the extruded film. The film may be repeatedly utilized as a transfer film without repeated embossing of the transfer film. Yet the transfer film disclosed by Watkins et al. lacks the desirable strength properties of oriented film.
Thus, it will be appreciated that there is a need for a process wherein a diffraction pattern may be embossed directly into an oriented polymeric film, preferably a wide film having a width of between 12 and 120 inches. Oriented films thus embossed may be employed as diffraction imaging materials themselves, and they can also function as transfer films or masters for the manufacture of diffraction imaging materials. As will be described in detail hereinbelow, the present invention provides for the embossing of complex diffraction patterns into durable, relatively hard, dimensionally stable, oriented polymeric films.